Sunlight Hampers Oil Dispersants

An aircraft sprays chemical dispersants onto a sunlight-weathered oil slick from the 2010 spill in the Gulf of Mexico.

Stephen Lehmann/Coast Guard

The 2010 Deepwater Horizon disaster spewed 795 million liters of crude oil into the Gulf of Mexico over a period of five months. One of the techniques used to mitigate the oil’s impact on the environment was the release of chemicals from airplanes flying above surface oil slicks. In all, 412 flights over a sixty-one-day period dropped nearly 3.7 million liters of chemical dispersants. The chemicals, known as surfactants, contain a combination of detergents and organic solvents, intended to help oil and water mix. This facilitates the formation of oil droplets that sink into the water column, where they are dispersed or may be ingested by microbes. Newly released analyses reveal, however, that the spraying was much less effective than expected.

The effectiveness of dispersants is known to decrease over time, as oil is weathered by the environment, but this weakening had previously been attributed to evaporation and emulsification rather than to any significant effect of sunlight. Earlier this year, a team of investigators led by Collin P. Ward of Woods Hole Oceanographic Institution in Massachusetts, determined that photochemical oxidation—the effect of sunlight on oil—affected more than half of the Deepwater Horizon’s floating oil spill within hours to days. In a more recently published study, Ward and his colleagues determined that sunlight altered not only the properties of crude oil but also the efficacy of the chemical clean-up effort.

The researchers took samples of unweathered oil collected from the spill site and artificially weathered them in the laboratory by allowing the oil to evaporate to varying degrees. Next, they exposed the weathered oil to simulated sunlight representative of that found in the Gulf of Mexico. For comparison, a control group of evaporated oil samples was kept in the dark. Sunlight had a far greater effect on the physical and chemical properties of the oil than did evaporation.

The team then tested the effectiveness of dispersants on dark-control and on sunlight-exposed oil floating in water. The chemicals were over thirty percent worse at dispersing sunlight-exposed oil from the water’s surface, compared to the oil kept in the dark. Additionally, longer sunlight exposure caused steeper drops in dispersant effectiveness.